Hot Water Extractable Organic Carbon Research Articles

Labile carbon and soil texture control nitrogen transformation in deep vadose zone

Understanding transient nitrogen (N) storage and transformation in the deep vadose zone is critical for controlling groundwater contamination by nitrate. The occurrence of organic and inorganic forms of carbon (C) and nitrogen and their importance in the deep vadose zone is not well characterized due to difficulty in sampling and the limited number of studies. We sampled and characterized these pools beneath 27 croplands with different vadose zone thicknesses (6–45 m). We measured nitrate and ammonium in different depths for the 27 sites to evaluate inorganic N storage. We measured total Kjeldahl nitrogen (TKN), hot-water extractable organic carbon (EOC), soil organic carbon (SOC), and δ13C for two sites to understand the potential role of organic N and C pools in N transformations. Inorganic N stocks in the vadose zone were 21.7–1043.6 g m−2 across 27 sites; the thicker vadose zone significantly stored more inorganic N (p < 0.05). We observed significant reservoirs of TKN and SOC at depths, likely representing paleosols that may provide organic C and N to subsurface microbes. The occurrence of deep C and N needs to be addressed in future research on terrestrial C and N storage potential. The increase of ammonium and EOC and δ13C value in the proximity of these horizons is consistent with N mineralization. An increase of nitrate, concurrent with the sandy soil texture and the water-filled pore space (WFPS) of 78 %, suggests that deep vadose zone nitrification may be supported in vadose zones with organic-rich layers such as paleosol. A profile showing the decrease of nitrate concentrations, concurrent with the clay soil texture and the WFPS of 91 %, also suggests denitrification may be an important process. Our study shows that microbial N transformation may be possible even in deep vadose zone with co-occurrence of C and N sources and controlled by labile C availability and soil texture.

The lower labile carbon of surface soils in Chinese semiarid areas

ABSTRACT Hot water extractable organic carbon (HWOC), the labile carbon component, is often used to indicate soil organic carbon (SOC) dynamics. Nevertheless, few studies have been carried out in arid climate areas which affects our full understanding of HWOC. Here, we investigated the change in HWOC in the topsoil of different ecosystems in the southern part of the Loess Plateau in the semiarid region of China and compared it with that in other regions. The HWOC concentrations of the study area (0-10 cm) were 0.27 ± 0.12 g C kg−1 and 0.19 ± 0.04 g C kg−1 in the natural and agricultural systems respectively, and the HWOC proportions were 1.38 ± 0.38% and 2.18 ± 0.22%. The HWOC concentration and proportion in the study area were much lower than the reported data in other areas, which may be affected by drought conditions. Irrigation could weaken the difference in HWOC between agricultural systems in different regions. Since HWOC is easily lost due to the impact of the arid climate, the soil carbon balance and carbon sequestration in arid and semiarid areas are relatively unstable, indicating that soil management should be improved in combination with water management.

Soil organic matter and geochemical characteristics shape microbial community composition and structure across different land uses in an Australian wet tropical catchment

AbstractSoil characteristics vary across land uses, and soil microbial communities are likely to respond to such variations. However, such responses are not well understood. In this study, we examined how specific changes in soil organic matter (SOM) and geochemical characteristics influenced the bacterial and fungal community composition and structure across land uses. Soil samples were collected from grazing, sugarcane, forest and banana land uses in the Johnstone River catchment (wet tropics), northeastern Australia. High‐throughput sequencing of 16S rRNA gene and ITS amplicons were employed to characterise soil bacterial and fungal communities, respectively. Bacterial and fungal α‐diversity did not differ across land uses, whereas strong shifts in their community composition and structure were evident. Patterns in microbial community composition were strongly (p &lt; 0.01) associated with changes in soil total organic carbon and nitrogen (TOC and TN), TOC:TN ratio, hot‐water extractable organic carbon (HWEOC) and soil geochemical characteristics (e.g., pH, Co, Mn, K, Mg and P contents). Distinct bacterial community composition was observed in forest soils, while fungal communities were similar in banana and forest soils compared with sugarcane and grazing. Overall, our results showed that SOM characteristics were the major driver on structuring soil microbial communities across land uses, while soil bacterial communities were more sensitive to variations in SOM and geochemical characteristics compared with fungi. Results from this study have implications for developing more targeted management practices across different land uses to improve soil health and ecosystem function in a wet tropical area.

Total and Hot-Water Extractable Organic Carbon and Nitrogen in Organic Soil Amendments: Their Prediction Using Portable Mid-Infrared Spectroscopy with Support Vector Machines

Against the background of climate change mitigation, organic amendments (OA) may contribute to store carbon (C) in soils, given that the OA provide a sufficient stability and resistance to degradation. In terms of the evaluation of OA behavior in soil, total organic carbon (TOC), total nitrogen (TN), and the ratio of TOC to TN (CN-ratio) are important basic indicators. Hot-water extractable carbon (hwC) and nitrogen (hwN) as well as their ratios to TOC and TN are appropriate to characterize a labile pool of organic matter. As for quickly determining these properties, mid-infrared spectroscopy (MIRS) in combination with calibrations based on machine learning methods are potentially capable of analyzing various OA attributes. Recently available portable devices (pMIRS) might replace established benchtop devices (bMIRS) as they have potential for on-site measurements that would facilitate the workflow. Here, we used non-linear support vector machines (SVM) to calibrate prediction models for a heterogeneous dataset of greenwaste composts and biochar compost substrates (BCS) (n = 45) using bMIRS and pMIRS instruments on ground samples. Calibrated models for both devices were validated on separate test sets and showed similar results. Ten OA were sieved to particle size classes (psc’s) of &gt;4 mm, 2–4 mm, 0.5–2 mm, and &lt;0.5 mm. A universal SVM model was then developed for all OA and psc’s (n = 162) via pMIRS. Validation revealed that the models provided reliable predictions for most parameters (R2 = 0.49–0.93; ratio of performance to interquartile distance (RPIQ) = 1.19–5.70). We conclude that (i) the examined parameters are sensitive towards chemical composition of OA as well as particle size distribution and can therefore be used as indicators for labile carbon and nitrogen pools of OA, (ii) prediction models based on SVM and pMIRS are a feasible approach to predict the examined C and N pools in organic amendments and their particle size class, and (iii) pMIRS can provide valuable information for optimized application of OA on cultivated soils at low costs and efforts.

Soil organic matter formation is controlled by the chemistry and bioavailability of organic carbon inputs across different land uses

Soil organic matter (SOM) formation involves microbial transformation of plant materials of various quality with physico-chemical stabilisation via soil aggregation. Land use and vegetation type can affect the litter chemistry and bioavailability of organic carbon (OC), and consequently influence the processing and stabilisation of OC into SOM. We used 13C nuclear magnetic resonance (13C NMR) and hot-water extraction to assess the changes in chemical composition and labile OC fractions during the transformation processes from leaf to litter to SOM depending on land use and vegetation type. The hot-water-extractable OC (HWEOC) decreased from leaf (43–65 g kg−1) to litter (19–23 g kg−1) to SOM (8–16 g kg−1) similar in four land use types: grassland, sugarcane, forest and banana. These trends demonstrated the uniform converging pathways of OC transformation and increasing stability by SOM formation. The preferential decomposition and decrease of labile OC fractions (∑% di-O-alkyl, O-alkyl and methoxyl) from leaf (54–69%) to SOM (41–43%) confirmed the increasing stability of the remaining compounds. Despite differences in the biochemical composition of the leaf tissues among the vegetation types, the proportions of labile OC fractions in SOM were similar across land uses. The OC content of soil was higher in forest (7.9%) and grassland (5.2%) compared to sugarcane (2.3%) and banana (3.0%). Consequently, the HWEOC per unit of soil weight was higher in forest and grassland (2.0 and 1.2 g kg−1 soil, respectively) compared to sugarcane and banana (0.3 and 0.4 g kg soil−1, respectively). The availability of labile SOM is dependent on the quantity of SOM not the chemical composition of SOM. In conclusion, labile OC fractions in SOM, as identified by 13C NMR, were similar across land use regardless of vegetation type and consequently, SOM formation leads to convergence of chemical composition despite diversity of OC sources.

Enzymatic Activity as New Moorsh-Forming Process Indicators of Peatlands

The aim of this study is to comprehensively assess the change in oxidoreductive enzyme activities, due to the potential in catalyzing oxidation and reduction reactions, as the basic processes on undrained and drained peat soils. On undrained peatlands, a significant decrease of enzyme activities was observed such as xanthine oxidase, urate oxidase, phenol oxidase, and peroxidase with an increase in depth. It was connected with significantly higher porosity values, hot water extractable organic carbon, and total organic nitrogen contents, ammonium and nitrate ions concentrations, and significantly lower ash and bulk density values in the upper layers. On drained peatlands, a significant increase of enzyme activities in depth was measured. Enzyme activities such as xanthine, urate, phenol oxidase, and peroxidase were documented to be effective as new indicators and tools for changes of the moorsh-forming process in association with the oscillation of the water table caused by the drainage of the peatlands.

Labile soil carbon and nitrogen fractions under short and long-term integrated crop–livestock agroecosystems

Context Labile soil C and N fractions are strongly influenced by agricultural management practices. Aims This study was conducted on three long-term (≥30 years) on-farm sites (sites 1–3), and one short-term (3 years) experimental site (site 4) to evaluate the impacts of integrated crop–livestock system (ICLS) on labile C and N fractions and β-glucosidase enzyme activity for the 0–5 cm soil depth. Methods Long-term management systems (ICLS, corn (Zea mays L.)/grazing–soybean (Glycine max (L.) Merr.)/grazing–cover crop/grazing), control (CNT, corn–soybean with no cover crop and no grazing), and a grazed pasture (GP) at sites 1–3 were compared. At site 4, the treatments included: ICLS (corn/grazing–soybean/grazing–oat (Avena sativa L.)–cover crop/grazing); CC (corn–soybean–oat–cover crop with no grazing); CNT (corn–soybean–oat–fallow with no grazing); and GP. Key results ICLS had higher hot water extractable organic carbon (HWC), cold water extractable organic carbon (CWC), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), potential carbon mineralisation (PCM) and potassium permanganate oxidisable carbon (POXC) than CNT under long-term management system. Long-term ICLS also enhanced β-glucosidase activity compared to CNT. ICLS had 72%, 214% and 60% higher glucosidase activity than the CNT at sites 1, 2 and 3, respectively. However, the C and N fractions and β-glucosidase activity were not affected by short-term ICLS (site 4). GP always had higher C and N fractions than ICLS and the CNT. There was a significantly positive relationship between SOC and labile C and N fractions, except POXC. Conclusions and implications Long-term inclusion of cover crops and livestock grazing in corn–soybean system was effective in enhancing labile soil C and N fractions.

Soil Organic Carbon and Labile Carbon Pools Attributed by Tillage, Crop Residue and Crop Rotation Management in Sweet Sorghum Cropping System

Labile organic carbon (LOC) fractions are considered as sensitive indicators of change in soil quality and can serve as proxies for soil organic carbon (SOC). Although the impact of tillage, crop rotation and crop residue management on soil quality is well known, less is known about LOC and SOC dynamics in the sweet sorghum production systems in South Africa. This short-term study tested two tillage levels: no-till and conventional-tillage, two crop rotations: sweet-sorghum/winter grazing vetch/sweet sorghum and sweet-sorghum/winter fallow/sweet sorghum rotations and three crop residue retention levels: 30%, 15% and 0%. Tillage was the main factor to influence SOC and LOC fractions under the sweet sorghum cropping system in South Africa. NT increased SOC and all LOC fractions compared to CT, which concurs with previous findings. Cold water extractable organic carbon (CWEOC) and hot water extractable organic carbon (HWEOC) were found to be more sensitive to tillage and strongly positively correlated to SOC. An increase in residue retention led to an increase in microbial biomass carbon (MBC). This study concludes that CWEOC and HWEOC can serve as sensitive early indicators of change in soil quality and are an ideal proxy for SOC in the sweet-sorghum cropping system in South Africa.

Investigation on labile organic carbons in soils developed from limestone and sandstone in karst areas of southwest China

Although labile soil organic carbons (LSOC) represent only a small part of soil organic carbon (SOC), they are an important source of energy for the belowground portion of the ecosystem and play a crucial role regarding changes in the quality of soil organic matter (SOM). This study measured and compared the content of LSOC and SOC in limestone soil (developed from limestone) and sandstone soil (developed from sandstone) in the karstic area of southwest China to better understand the impacts of different soil types on LSOC (water-extractable and readily oxidizable organic carbon) and evaluate the existence of a possible relationship between LSOC and SOC in the soil. The results demonstrated that the amounts of SOC, soil readily oxidizable carbon (SROC), and soil water-extractable organic carbon (WEOC) (hot-water extractable organic carbon (hot-WEOC) and cold-water extractable organic carbon (cold-WEOC)) were the highest in the surface soil and decreased with increasing depth in both types of soil profiles. In addition, the content of SOC and LSOC were significantly higher in the limestone soil profiles than those in the sandstone soil profiles. This may be as a result of the occurrence of higher pH values and higher content of calcium, magnesium, and clay minerals in the limestone soil than in the sandstone soil. The proportions of WEOC in total SOC extracted at 25°C and 70°C in the sandstone soil profiles displayed higher average values of 0.040% and 0.527%, respectively, whereas the limestone soil profiles displayed lower average values of 0.015% and 0.255% for the WEOC extracted at 25°C and 70°C, respectively. Regarding the SROC content, both soil profiles showed higher values than WEOC content, ranging from 39.4%–55.2%. The results also showed that for the two types of soils significant correlations were found between samples of SROC, WEOC, and SOC (P < 0.01). Among the LSOC, the hot-WEOC showed clear differences between different soil types, and significant relationships were observed for not only SOC but also SROC. The correlation coefficients between hot-WEOC and SOC or SROC were higher than those between cold-WEOC and SOC or SROC. This study, therefore, suggests that hot-WEOC could be a useful indicator of changes in the quality of SOM in different soil types.

Soil organic carbon and associated soil properties in Enset (Ensete ventricosum Welw. Cheesman)-based homegardens in Ethiopia

Enset (Ensete ventricosum Welw. Cheesman)-based homegardens have long been practiced as central elements of agricultural land management and food security in south and southwest Ethiopia. In contrast to the homegardens’ biodiversity and role in food security, soil quality has received little attention. Objective of this study was to assess soil quality parameters in typical homegardens in comparison with adjacent croplands, both under continuous management for >30 years. The study was undertaken at high (2200–2330 masl), mid (1799–1849 masl), and low (1349–1381 masl) elevation in the central Omo-Gibe basin, southwest Ethiopia. Through interviews of 49 randomly selected farm households, and soil sampling at six paired sites at high and mid elevations, and five paired sites of low elevation, we found that homegardens received the majority of household waste and manure and were less frequently tilled. By contrast, some, but not all, croplands occasionally received inorganic fertilizer. Homegarden soil had significantly greater (P < 0.05) soil organic carbon (SOC) concentrations than croplands. At 0–20 cm depth, SOC concentrations in homegardens (22.4–26.4 mg g−1 soil) were twice as high as in croplands (11.5–12.7 mg g-1 soil). Most likely the lower content of SOC in cropland was due to the limited input of organic matter and intensive tillage. The top 60 cm of homegarden soils stored 21–32 Mg ha-1 more SOC than adjacent croplands. Homegardens at high elevation had a significantly greater SOC stock (P < 0.05) than at low elevation. Hot water extractable (labile) organic carbon levels at 0–20 cm in homegardens (540–649 μg g-1 soil) were three to five times greater than in croplands (106–207 μg g-1 soil) and was strongly correlated with the SOC concentration (R2 = 0.85, in homegardens). The fraction of water-stable macro-aggregates (>0.5 mm) was positively correlated with the SOC concentration and significantly greater in homegardens than in cropland. Our results show that traditional homegardens represent a sustainable form of land management and cropping system, enhancing SOC concentration, soil structure and fertility.

Changes in soil organic matter stability with depth in two alpine ecosystems on the Tibetan Plateau

Soil organic carbon (SOC) decomposition can potentially feedback to climate change. However, the biotic, abiotic and inherent factors controlling the stability of soil carbon, and changes in these factors with soil depth, remain poorly understood. In this study, we combined a number of complementary methods to quantify the biological, thermal, chemical, molecular and isotopic indices of soil organic matter (SOM) stability along the soil profile (0–70 cm) in two contrasting alpine ecosystems (meadow and shrubland) on the Tibetan Plateau. Firstly, we conducted an aerobic lab–incubation experiment on root–free, sieved soils. The number of days to respire 5% of initial SOC, a biological index of SOM stability, decreased with soil depth. Moreover, the temperature at which half of SOM mass loss (TG–T50), a thermal index of SOM stability, increased with soil depth. Additionally, hot–water extractable organic carbon (HWEOC) per gram SOC, a chemical index of SOM stability, showed weak (meadow) and little (shrubland) declining trend with depth. Further, we used Fourier–transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopy to characterize the molecular composition of SOM. The index of recalcitrance of FTIR spectra and the combined index of aliphaticity and aromaticity of NMR spectra both increased with depth, suggesting that the molecular composition of SOM was more complex with increasing depth. Finally, the isotopic values of SOM (13C and 15N) and the 14C–based SOC turnover time both increased with depth, indicating that the isotopic indices of SOM stability also increased with depth. Overall, our results suggest that the thermal, chemical, molecular and isotopic indices of SOM stability were mutually correlated and all showed increasing trend with increasing soil depth in the two alpine ecosystems, although the biological index (as measured by aerobic incubation of root–free sieved soils) showed the opposite results.

Effects of straw management and nitrogen application rate on soil organic matter fractions and microbial properties in North China Plain

A 2-year field experiment was conducted on the North China Plain to assess the effects of two straw management practices and different nitrogen fertilizer addition levels on soil organic matter (SOM) fractions and microbial properties under a winter wheat-summer maize cropping system. Eight treatments (two straw management practices and four nitrogen fertilizer input levels) were established using a randomized complete block design. The straw management practices were no straw return and straw return. Each straw management practice received nitrogen fertilizer rates at 0 (N0), 270 (N270), 360 (N360), or 450 (N450) kg N ha−1 year−1. The soil properties measured included SOM fractionation, enzyme activities, community level physiology profile (CLPP), and microbial community abundance and composition, which were represented by phospholipid fatty acid (PLFA). Compared to no straw return, the addition of straw significantly increased soil total organic carbon and nitrogen, particulate organic carbon and nitrogen, and occluded particulate organic carbon and hot water extractable organic carbon by 7–29%. Invertase, protease, urease, and dehydrogenase activities increased by 22–40%. The biomasses of Gram-positive bacteria and fungi also increased. The increase in SOM fractions may be attributed to the decomposition of the added straw by more active enzymes, which may result from the shift of microbial community composition. In addition, hot-water extractable organic carbon, NO3-N, urease activity, and fungal biomass in the N270, N360, and N450 treatments significantly increased, independent of the nitrogen application rate, which indicated that N input level had a non-linear effect on soil properties in this area. Short-term straw return combined with a modest nitrogen fertilizer input (270 kg N ha−1 year−1) can increase SOM contents and microbial activity, which may help improve soil fertility in this region.

Labile soil organic carbon for assessing soil quality: influence of management practices and edaphic conditions

Soil organic carbon (SOC) is a complex set of pools, and to understand its dynamics it is necessary to know which of these pools are sensitive to the edaphic and climatic conditions or the agricultural practices, or to both. The objectives of this study were to evaluate the relationships between permanganate oxidizable C (POX-C) and various soil organic carbon fractions in different land-uses and soil types, and to examine whether the POX-C fraction is sensitive to different agricultural management practices in soils under no-tillage. Three treatments were identified at four sites located in the Argentine Pampas region: two different agricultural scenarios in terms of crop rotation, fertilizers and use of agrochemicals (Good Agricultural Practices and Poor Agricultural Practices, GAP and PAP, respectively) and an undisturbed natural (NE) environment adjacent to the agricultural sites as the control treatment. The following organic fractions were quantified: SOC, coarse and fine particulate organic carbon (POCc and POCf, respectively), hot water and acid extractable organic carbon (HWC and HAC, respectively) and POX-C. Soil POC values ranged from 0.46 to 7.29 g kg−1, HAC values ranged from 1.50 to 6.73 g kg−1, HWC values ranged from 0.20 to 1.10 g kg−1 and POX-C values ranged from 0.41 to 1.04 g kg−1 soil, POCc being the most variable fraction (CV = 72%) and POX-C the least (CV = 22%). Soil POCc and POCf at 0–10 cm, and POCc at 10–20 cm were largely explained by management practices with a component of variance >50%. The relationship between POX-C and SOC was generally stronger (R2 = 0.76–0.92) than POX-C with other organic fractions and where depth and site factors have a greater influence on this relationship than management practices. Among the labile fractions, the most sensitive indicators of soil quality in agricultural soils were POCf and HWC, which displayed the highest F-statistic values. Despite the dilute solution used (0.02 mol L−1 KMnO4) the POX-C demonstrated limited sensitivity to different agricultural practices. However, this methodology could be used to estimate SOC regarding site conditions and depths. The POCf was the fraction most affected by agricultural practices, indicated by high relationships with both the soil physical attributes (macroporosity, bulk density, and density, volume and stability of aggregates) and the agronomic parameters (soybean and maize yields).

Assessing the vulnerability of organic matter to C mineralisation in pasture and cropping soils of New Zealand

In New Zealand, pastoral soils have substantial organic carbon (OC) stocks, which may be vulnerable to loss from disturbance and environmental perturbations. We assessed OC vulnerability using two approaches. For the first approach, we postulated that the OC deficit of continuously cropped soils relative to nearby pastoral soils would provide a measure of the quantity of potentially vulnerable OC in pastures. As a test, soils were sampled to a depth of 15 cm at 149 sites and the total organic carbon (TOC) and particulate organic carbon (POC) contents were measured. The second approach involved measurement of OC mineralisation in a laboratory assay (98 day aerobic incubation at 25°C). For the pastoral soils, the mean TOC and POC was about twice that of the cropped soils. On average, 89% more OC was mineralised from the pastoral soils compared with the cropped counterparts. However, the quantity of OC mineralised in pasture soils was small relative to the potential for OC loss inferred from the difference in TOC between pastoral and cropped soils. Carbon mineralisation was explained using a two-pool exponential model with rate constants of the ‘fast’ and ‘slow’ pools equating to 0.36 ± 0.155 and 0.007 ± 0.003 day–1 respectively. The larger, slow OC pool correlated strongly with hot water extractable OC whereas the fast pool was related to OC extracted using cold water. Our results suggest that water extraction (using cold and hot water) can provide a rapid estimate of the quantity of mineralisable OC across a wide range of New Zealand soils.

Liming improves soil microbial growth, but trash blanket placement increases labile carbon and nitrogen availability in a sugarcane soil of subtropical Australia

Liming has been widely used to decrease soil acidity, but its effects on soil nitrogen (N) availability and microbial processes in sugarcane fields are largely unknown. Adjacent sugarcane soils at 26 months after liming (26ML), 14 months after liming (14ML) and with no lime amendment (CK) in Bundaberg, Australia, were selected to investigate the effect of liming on soil N bioavailability and microbial activity in a long-term subtropical sugarcane cropping system. Liming in both 14ML and 26ML treatments significantly increased soil pH (by 1.2–1.4 units) and exchangeable Ca2+ (&gt;2-fold) compared with the CK treatment. The lower concentrations of hot water extractable organic carbon (C) and total N and ammonium-N in the 14ML, compared with the CK and 26ML treatments, can be attributed to the absence of trash blanket placement in the former. Enhanced microbial immobilisation due to improved soil pH by liming (14ML and 26ML treatments) led to increased soil microbial biomass C and N, particularly in the presence of a trash blanket (26 ML treatment), but decreased soil respiration and metabolic quotient indicated that acidic stress conditions were alleviated in the liming treatments. Soil pH was the main factor governing soil enzyme activities, with an overall decrease in all enzyme activities in response to liming. Overall, liming and trash blanket practices improved sugarcane soil fertility. Further study is warranted to investigate the shifts in soil microbial community composition and the diversity and abundance of N-associated functional genes in response to liming in sugarcane fields.

Soil carbon fractions and biological activity based indices can be used to study the impact of land management and ecological successions

Soil organic carbon (SOC) is a strong indicator of soil health. Development of efficient soil quality indicators is crucial to better understand the impact of land management strategies on the recovery of degraded ecosystems. We hypothesized that SOC fractions and biological attributes can compose strong soil quality indicators to assess an ecosystem recovery following disturbance. Thus, the objective of this study was to evaluate the use of soil biological activity and SOC fractions to study the impact of different land use systems and ecological successions in ecosystem recovery. We selected six land use systems: tobacco (Nicotiana tabacum) cultivation; pastureland; reforested land with Eucalyptus sp.; and natural ecological successions with 10, 20 and 35 years of vegetation regeneration, respectively. We collected disturbed and undisturbed soil samples in triplicate at 0–5, 5–10, 10–20 and 20–40cm depth intervals. Several fractionation approaches were used to determine SOC pools: hot water extractable organic carbon, permanganate oxidized organic carbon, particulate organic carbon, mineral associated organic carbon and total SOC. The activity of the enzyme arylsulfatase was used to represent soil biological attributes. We calculated three indices to represent the soil quality: carbon management index, soil resilience index and biological activity index. Our results suggest that the SOC fractions and the enzyme activity followed the increase of vegetation complexity of the ecological succession stages. The labile SOC pool, in addition to enzyme activity, was the most sensitive variable to assess land use changes. The biomass-C input was considered to be the main reason of SOC increase, and the gains of labile SOC fractions were directly related to the increase of SOC stocks. Both, biological and carbon management indices were efficient tools to characterize the impact of studied management systems. Also, we found that assessment of deeper soil layers (20–40cm) was extremely important as incomplete inferences might be reached while evaluating only surface soil layers (0–20cm). We conclude that the carbon management and biological indices captured the stage of soil degradation and the influence of vegetation diversity in the soil resilience restoration, providing an advance in monitoring strategies that can be reproducible in any environment.

Effects of residue incorporation and plant growth on soil labile organic carbon and microbial function and community composition under two soil moisture levels

This study investigates the effects of residue incorporation coupled with plant growth and soil moisture level on wheat biomasses, soil nutrients, labile organic carbon (LOC), microbial metabolic profiles, and community composition. Four management practices were used in a 180-day pot experiment: (1) control (CON), (2) maize (Zea mays L.) residue incorporation without plants (MR), (3) wheat (Triticum aestivum L.) plants without maize residue (WP), and (4) maize residue incorporation with wheat plants (MRWPs). Each management practice included soil moisture at both 40 and 80% of field capacity. At wheat harvest, soil nutrient contents in the WP and MRWP treatments were significantly lower than in the CON and MR treatments. In comparison with the CON treatment, MR, WP, and MRWP treatments resulted in 35, 23, and 67% increases in dissolved organic carbon content; 17, 12, and 34% increases in hot-water extractable organic carbon content; and 78, 50, and 150% increases in microbial biomass carbon content. Furthermore, microbial utilizations of carboxylic acids and polymer carbon sources in the MR, WP, and MRWP treatments were 261 and 88%, 239 and 105%, and 300 and 126% higher than in the CON treatment. The MR and CON treatments had similar phospholipid fatty acid (PLFA) content but the WP and MRWP treatments had significantly increased gram-negative content and changes to community composition compared with the CON and MR treatments. The wheat biomass, LOC, and PLFA contents significantly increased with greater soil moisture. Overall, these results suggest an additive effect of residue incorporation and plant growth on LOC contents, primarily due to the changes in microbial utilization of carbon sources and community composition.

Effects of Maize Residue Quality and Soil Water Content on Soil Labile Organic Carbon Fractions and Microbial Properties

Investigating the effects of residue chemical composition on soil labile organic carbon (LOC) will improve our understanding of soil carbon sequestration. The effects of maize residue chemical composition and soil water content on soil LOC fractions and microbial properties were investigated in a laboratory incubation experiment. Maize shoot and root residues were incorporated into soil at 40% and 70% field capacity. The soils were incubated at 20 °C for 150 d and destructive sampling was conducted after 15, 75, and 150 d. Respiration, dissolved organic carbon (DOC), hot-water extractable organic carbon (HEOC), and microbial biomass carbon (MBC) were recorded, along with cellulase and β-glucosidase activities and community-level physiological profiles. The results showed that the cumulative respiration was lower in root-amended soils than in shoot-amended soils, indicating that root amendment may be beneficial to C retention in soil. No significant differences in the contents of DOC, HEOC and MBC, enzyme activities, and microbial functional diversity were observed between shoot- and root-amended soils. The high soil water content treatment significantly increased the cumulative respiration, DOC and HEOC contents, and enzyme activities compared to the low soil water content treatment. However, the soil water content treatments had little influence on the MBC content and microbial functional diversity. There were significantly positive correlations between LOC fractions and soil microbial properties. These results indicated that the chemical composition of maize residues had little influence on the DOC, HEOC, and MBC contents, enzyme activities, and microbial functional diversity, while soil water content could significantly influence DOC and HEOC contents and enzyme activities.

Soil labile organic carbon and carbon-cycle enzyme activities under different thinning intensities in Chinese fir plantations

Thinning is a silvicultural tool that is used to facilitate the growth of timber plantations worldwide. Plantations are important CO2 sinks, but the mechanism by which thinning affects the quantity and stability of soil organic carbon (SOC) is poorly understood. In this study, we examined the effects of different thinning intensities (low-intensity thinning treatment with 30% of the trees removed; high-intensity thinning treatment with 70% of the trees removed; control treatment without tree removal) on the quantity and stability of SOC in Chinese fir (Cunninghamia lanceolata [Lamb.] Hook) plantations in southeastern China. The amounts of SOC, microbial biomass carbon (MBC), easily oxidizable carbon (EOC), cold-water- soluble organic carbon (CWSOC) and hot-water- extractable organic carbon (HWEOC) and the carbon-cycle-related enzyme activities (β-glucosidase, invertase and cellulose) were quantified. We found that thinning significantly decreased the amount of SOC compared with the control treatment, but the effect differed by sampling date. The MBC and EOC were significantly higher in the high-intensity thinning treatment than in the control and low-intensity thinning treatments, whereas the invertase and β-glucosidase activities were significantly higher in the control treatment. However, the amounts of CWSOC, HWEOC and cellulose activity did not differ among the treatments, which indicates that the MBC, EOC and the activities of invertase and β-glucosidase were better indicators of changes in SOC to thinning. In addition, the MBC, EOC, CWSOC and the β-glucosidase and cellulase activities peaked in the warmer months. Our results indicate that thinning treatments in Chinese fir plantations decreased the SOC quantity and enzyme activities and that high-intensity thinning may lead to an increase of labile SOC.

The sensitivity of water extractable soil organic carbon fractions to land use in three soil types

ABSTRACTSoil organic carbon (SOC) has a high impact on the sustainability of ecosystems, global environmental processes, soil quality and agriculture. Long-term tillage usually leads to SOC depletion. The purpose of this study was to determine the impact of different land uses on water extractable organic carbon (WEOC) fractions and to evaluate the interaction between the WEOC fractions and other soil properties. Using an extraction procedure at 20°C and 80°C, two fractions were obtained: a cold water extractable organic carbon (CWEOC) and a hot water extractable organic carbon (HWEOC). The results suggest that there is a significant impact from different land uses on WEOC. A lower relative contribution of WEOC in SOC and a lower concentration of labile WEOC fractions are contained in arable soil compared to forestlands. Chernozem soil was characterized by a lower relative contribution of WEOC to the SOC and thus higher SOC stability in contrast to Solonetz and Vertisol soils. Both CWEOC and HWEOC are highly associated with SOC in the silt and clay fraction (<53 µm) and were slightly associated with SOC in the macroaggregate classes. The WEOC fractions were highly and positively correlated with the SOC and mean weight diameter.